US city splits westbound in tailspin (6)
I believe the answer is:
spiral
'tailspin' is the definition.
(I've seen this in another clue)
'us city splits westbound' is the wordplay.
'us city' becomes 'LA' (Los Angeles).
'splits' becomes 'rips' ('rip' can be a synonym of 'split').
'westbound' shows that the letters should be reversed in order (letters written from right to left).
'la'+'rips'='larips'
'larips' in reverse letter order is 'SPIRAL'.
'in' acts as a link.
(Other definitions for spiral that I've seen before include "design for some staircase" , "Ever widening curve" , "Sir, pal winds in continuous loops" , "Corkscrew curve" , "Mushroom" .)